In the Comstock Statue room where Booker is ambushed by some enemies, enter the workshop where the enemies came from to find this phial stored in the back. A destroyed Cornelius Slate statue will be in this workshop.

When fighting Slate's men in the Courtyard, face the message on the wall that reads "We deserve to die as soldiers" -- and turn 90° left. Go through the door and into the workshop and locate the phial in the toolbox near the voxophone (not shown above). You may want to do this after you deal with the motorized patriot.